The present invention relates to cam phasers for altering the phase relationship between valve motion and piston motion in reciprocating internal combustion engines; more particularly, to cam phasers having a vaned rotor rotatably disposed in an internally-lobed stator wherein the rotor and stator can be mechanically locked together by a locking pin; and most particularly where the dimensions of the locking pin and locking pin guide are optimized to prevent binding of the pin in operation.
Cam phasers are well known in the automotive art as elements of systems for reducing combustion formation of nitrogen oxides (NOX), reducing emission of unburned hydrocarbons, improving fuel economy, and improving engine torque at various speeds. Typically, a cam phaser employs a first element driven in fixed relationship to the crankshaft and a second element adjacent to the first element and mounted to the end of the camshaft in either the engine head or block. A cam phaser is commonly disposed at the camshaft end opposite the engine flywheel. The first element is typically a cylindrical stator mounted onto a crankshaft-driven gear or pulley, the stator having a plurality of radially-disposed inwardly-extending spaced-apart lobes and an axial bore. The second element is a vaned rotor mounted to the end of the camshaft through the stator axial bore and having vanes disposed between the stator lobes to form actuation chambers therebetween such that limited relative rotational motion is possible between the stator and the rotor. Such a phaser is known in the art as a vane-type cam phaser.
The disposition of the rotor in the stator forms a first, or timing-advancing, array of chambers on first sides of the vanes and a second, or timing-retarding, array of chambers on the opposite sides of the vanes. The apparatus is provided with suitable porting so that hydraulic fluid, for example, engine oil under engine oil pump pressure, can be brought to bear controllably on opposite sides of the vanes in the advancing and retarding chambers. Control circuitry and valving, commonly a multiport spool valve, permit the programmable addition and subtraction of oil to the advance and retard chambers to cause a change in rotational phase between the stator and the rotor, in either the rotationally forward or backwards direction, and hence a change in timing between the pistons and the valves.
Under conditions of low engine oil pump pressure, such as during startup, it is desirable to mechanically lock the rotor and stator together in a default mode to prevent unwanted relative angular movement of the rotor/stator when the pump pressure is not high enough to reliably position the rotor relative to the stator. This is typically accomplished by a hydraulically activated locking pin disposed in the rotor and positioned parallel to the rotational axis of the phaser. In the default position, when the rotor and stator are locked together, a spring biases a cylindrical locking pin outward to engage a pin bore disposed in the stator. When the oil pump pressure reached a pre-determined level, the hydraulic force of the oil causes the locking pin to retract from the pin bore and into the rotor thereby mechanically decoupling the rotor from the stator and permitting cam shaft phasing to occur. When the rotor and stator are locked together in the default mode, the torsional forces applied to the stator by the engine crankshaft are transferred to the rotor/camshaft via lateral loading of the locking pin in the pin bore. This means that, while it is desirable for the pin to be retracted from the coupled mode in a smooth and predictable manner, the additional and irregularly applied frictional bias caused by the lateral loading of the locking pin results in pin retraction and the decoupling event to occur erratically.
What is needed is a means for reducing the frictional bias caused by the lateral loading of the locking pin to permit a more precise control of the oil pump pressure at which the pin is retracted from the pin bore and at which mechanical decoupling of the stator and rotor can occur.
The present invention is directed to a vane-type camshaft phaser wherein a locking pin assembly is disposed between a rotor and a stator of the phaser to selectively couple the rotor and stator together. The central axis of the locking pin assembly is parallel to the rotational axis of the phaser. The pin is guided through its axial movement by two cylindrical guide surfacesxe2x80x94an inner guide surface and an outer guide surface. The lengths of these guide surfaces are optimized to minimize binding and sluggish operation of the pin caused by lateral forces exerted on the pin when in operation. The outer guide surface to inner guide surface ratio (L/I) is preferably greater than 2.